The single most fundamental limitation to the signal bandwidth that can be transmitted on optical fibers is chromatic dispersion of the modulated signal in the fiber. In essence, an amplitude modulated optical signal will usually have two information sidebands, one above and the other below the optical frequency of the original light source. The frequencies associated with these sidebands will travel down an optical fiber at different velocities with the result that different parts of the spectrum will arrive at the far end at different times. Thus, if digital signals are sent as narrow pulses, these pulses will be broadened or dispersed, eventually overlapping with neighbouring pulses.
Approaches currently used to reduce the effects of chromatic dispersion include (1) reversing the effects of chromatic dispersion in the optical domain, (2) reversing the effects in the electrical domain and (3) reducing the transmission bandwidth of the optical signal on the fiber. The first is based on purely optical methods where the effects of group velocity dispersion are reversed while the signal is still in the optical domain. Adding dispersion compensating fiber in the transmission path is a common approach. Other optical methods include compensation by differential time delay of the upper and lower sidebands of the modulated signal, see A. Djupsjobacka, O. Sahlen, "Dispersion compensation by differential time delay," IEEE Journal of Lightwave Technology, vol. 12, no. 10, pp. 1849-1853, October 1994; spectrally inverting the signal at the midpoint of the transmission path, see R. M. Jopson, A. H. Gnauck, R. M. Derosier, "10 Gb/s 360-km transmission over normal-dispersion fiber using mid-system spectral inversion," Proceedings OFC'93, paper PD3, 1993; and prechirping the transmitted signal in an external modulator, see F. Koyoma, K. Iga, "Frequency chirping in external modulators," IEEE Journal of Lightwave Technology, vol. 6, no. 1, pp. 87-93, January 1988 and A. H. Gnauck, S. K. Korotky, J. J. Veselka, J. Nagel, C. T. Kemmerer, W. J. Minford, D. T. Moser, "Dispersion penalty reduction using an optical modulator with adjustable chirp," IEEE Photonics Technology Letters, vol. 3, no. 10, pp. 916-918, October 1991.
The second approach, in which dispersion effects are reversed in the electrical domain, is based on coherent transmission and heterodyne detection followed by equalization in the electrical domain. It is important to note that a double sideband (DSB) signal must be heterodyne detected if the signal is to be compensated electrically. If homodyne detection were attempted with a DSB signal, the upper and lower sidebands would overlap upon detection and the phase information would be lost and the higher modulation frequencies severely attenuated or distorted through cancellation of sideband components. Some techniques used or proposed for post-detection equalization include microstrip lines, see K. Iwashita, N. Takachio, "Chromatic dispersion compensation in coherent optical communications," Journal of Lightwave Technology, vol. 8, no. 3, pp. 367-375, March 1990; microwave waveguides, see J. H. Winters, "Equalization in coherent lightwave systems using microwave waveguides," Journal of Lightwave Technology, vol. 7, no. 5, pp. 813-815, May 1989. and fractionally spaced equalizers, see J. H. Winters, "Equalization in coherent lightwave systems using a fractionally spaced equalizer," Journal of Lightwave Technology, vol. 8, no. 10, pp. 1487-1491, October 1990.
The third approach is to modify the transmission format where the baseband signal spectrum is compressed. These types of approaches, which reduce the transmission bandwidth required on the fiber to transmit a given bit rate, are generally implemented by modifying the line code format in order to reduce the effective bandwidth required to transmit or receive the data, see K. Yonenaga, S. Kuwano, S. Norimatsu, N. Shibata, "Optical duobinary transmission system with no receiver sensitivity degradation," Electronics Letters, vol. 31, no. 4, pp. 302-304, February 1995, and G. May, A. Solheim, J. Conradi, "Extended 10 Gb/s fiber transmission distance at 1538 nm using a duobinary receiver," IEEE Photonics Technology Letters, vol. 6, no. 5, pp. 648-650, May 1994.
The generation, transmission and detection of single sideband (SSB) signals has been used in the RF and microwave regions of the electromagnetic spectrum to reduce the bandwidth of the signal by a factor of two, by sending either the upper or the lower sideband. Generation and transmission of SSB optical signals is shown in M. Izutsu, S. Shikama, T. Sueta, "Integrated optical SSB modulator/frequency shifter," IEEE Journal of Quantum Electronics, vol. QE-17, no. 11, pp. 2225-2227, November 1981 with a Mach Zender by and R. Olshansky, "Single sideband optical modulator for lightwave systems," U.S. Pat. No. 5,301,058, 1994. A dispersion benefit accrues from a single sideband signal due to the fact that the transmitted signal spectrum has been reduced by a factor of two.
A more significant advantage of optical SSB transmission is that upon detection, particularly if the signal is coherently detected, a dispersed baseband signal is produced where the information of the relative arrival time of the various signal frequencies remains as part of the electrical output signal and hence the fiber dispersion can be compensated in the electrical domain after detection. This advantage is similar to that for heterodyne detection of DSB signals, but with SSB transmission and detection, the signal can be homodyned directly to baseband using carrier signal added either at the source or at the receiver and thus it can be directly detected with the phase or delay information of the transmitted signal preserved.
An early integrated optical SSB modulator using optical filtering techniques was described in K. Yonenaga, N. Takachio, "A Fiber chromatic dispersion compensation technique with an optical SSB transmission in optical homodyne detection systems," IEEE Photonics Technology Letters, vol. 5, no. 8, pp 949-951, August 1993, where the first optical VSB transmission of baseband digital signals was carried out." In K. Yonenaga, N. Takachio, "Dispersion compensation for homodyne detection systems using a 10 Gb/s optical PSK-VSB signal," IEEE Photonics Technology Letters, vol. 7, no. 8, pp. 929-931, August 1995, the VSB design previously published was improved upon. In R. Olshansky's design, a single sideband optical modulator was described for the purpose of transmitting two or more optical signals with different optical carrier frequencies on a single fiber. The purpose of transmitting the signals in a single sideband format is to permit these optical carrier frequencies to be spaced as closely as the maximum modulation frequency. In one of the variations described in Olshansky, each optical SSB signal was generated with a single Mach-Zehnder (MZ) modulator. The input electrical information was modulated on RF sine and cosine carriers for the purpose of driving each arm of the dual-arm drive MZ modulator. By modulating the input signals on RF carriers, the application can be classified as a narrowband one since the bandwidth of the information signal carried by each RF carrier is small compared to the RF carrier frequency. When an information signal is used to narrowband modulate a sinewave carrier, the Hilbert transform of this modulated sinewave carrier is equivalent to a cosine carrier modulated by the original information signal. Further work on generation of SSB optical signals with a Mach-Zehnder modulator is found in Olshansky, U.S. Pat. no. 5,301,058.